Field of the Invention
This invention relates generally to lighting, and more particularly, to ceramic discharge chambers for a lamp, such as a ceramic metal halide lamp or a high pressure sodium discharge lamp. This invention also relates to a method of manufacturing ceramic arc chambers.
Discharge lamps produce light by ionizing a fill such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as a “dose”, emits a desired spectral energy distribution in response to being excited by the electric arc.
The discharge chamber in a discharge lamp can be formed from a vitreous material such as fused quartz, which is shaped into a desired chamber geometry after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, at temperatures greater than about 950 to 1,000° C., the halide fill reacts with the glass to produce silicates and silicon halide, reducing the quantity of fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall. These fill depletions cause color shift over time, which reduces the useful life of the lamp.
Ceramic discharge chambers were developed to operate at high temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. U.S. Pat. Nos. 4,285,732 and 5,725,827, for example, disclose translucent polycrystalline sintered bodies where visible wavelength radiation is sufficiently able to pass through to make the body useful for use as an arc tube.
Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder and then sintered together. For example, referring now to European Patent Application No. 0587238, five ceramic parts are used to construct the discharge chamber of a metal halide lamp. Two end plugs with a central bore are fabricated by die pressing a mixture of a ceramic powder and binder. A central cylinder and the two legs are produced by extruding a ceramic powder/binder mixture through a die. After forming the part, it is air sintered between 900–1400° C. to remove organic processing aids. Assembly of the discharge chamber requires tacking of the legs to the cylinder plugs, and the end plugs into the end of the central cylinder. This assembly is then sintered to form joins which are bonded by controlled shrinkage of the individual parts.
Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder. For example, end plugs with the central bore may be fabricated by die pressing a mixture comprising a ceramic powder and an organic binder. A central cylinder, and the two legs may be produced by extruding a ceramic powder/binder mixture through a die. Assembly of the discharge chamber involves the placement and tacking of the legs to the end plugs and the end plugs into the ends of the central cylinder. This final assembly is then sintered to form four joins which are bonded by controlled shrinkage of the individual parts.
The conventional ceramic discharge chamber method of construction has a number of disadvantages. For example, the number of component parts is relatively large and introduces the corresponding number of opportunities for variation and defects. Also, the conventional discharge chamber includes four bonding regions, each of which introduces an opportunity for lamp failure by leakage of the fill material if the bond is formed improperly. Each bonding area also introduces a region of relative weakness, so that even if the bond is formed properly, the bond may break during handling or be damaged enough in handling to induce failure in operation.
Another disadvantage relates to the precision with which the parts can be assembled and the resulting effect in the light quality. It is known that the light quality is dependent to a substantial extent on the voltage across the electrode gap, which in turn requires the size of the gap to consistently fall within an acceptable tolerance. Preferably, this result is achieved without significant effort devoted to optimizing the manufacturing process. However, divergent shrinkage rates of variously shaped components limit the ability to manufacture in a reliable manner. Accordingly, it would be desirable to minimize the component parts necessary to manufacture the ceramic arc chamber.